Dietary fat is the most concentrated source of energy of all the nutrients, typically supplying about 9 kcal/gram, which generally exceeds the caloric content provided by either dietary carbohydrates or protein. Fat contributes to the palatability and flavor of food, since most food flavors are fat-soluble, and to the satiety value, since fatty foods remain in the stomach for longer periods of time than do foods principally containing protein and carbohydrate. Furthermore, fat is a carrier of the fat-soluble vitamins A, D, E, and K and the essential fatty acids, which have been shown to be important in growth and in the maintenance of many body functions. Major research efforts have focused on ways to produce food substances that provide similar functional and organoleptic properties as fats at reduced caloric content but which are not readily perceived as being synthetic by consumers.
The most abundant group of fats are triglyceride-esters of fatty acids with glycerol (1,2,3-propanetriol). The health benefits of dietary omega 3, 6, and 9 fatty acids have been widely reported. See, e.g., Kennedy, “Structured Lipids: Fats of the Future,” Food Technology, November 1991, 76-83. Natural fats have a broad range of functionalities and are handled in different ways by the human digestive process. Early studies reported that triglyceride fats having high melting points were less digestible (Deuel, The Lipids, Vol. II, Interscience Publishers, 1955, pages 218 to 220). Later investigators questioned the relationship between digestibility and melting points, and scrutinized instead chain length and degree of unsaturation of fatty acid substituents. Using a rat model, straight chain, saturated fatty acids having 4 to 10 carbon atoms were completely digested, those having 10 to 18 carbons progressively less digested, and those having 18 or higher only slightly absorbed; and monounsaturated acids were about the same as saturated acids (Carroll, J. Nutr. 64: 399-410 (1957) at 408).
Other prior triglyceride metabolic studies found only limited areas of predictability. In one human study, a coconut oil fraction containing predominantly saturated, long chain triglycerides bearing 89 percent stearic (C18) and 11 percent palmitic (C16) acid residues provided 31 percent absorption, as compared to 98 percent for corn oil (Hashim et al., Am. J. Clin. Nutr. 31: S273-276 (1978)). However, it was found that increasing the stearic acid content of dietary fat did not per se decrease absorbability; rather, absorbability could be decreased by increasing the amount of tristearin present (i.e., triglycerides having three stearic residues; see Mattson, J. Nutr. 69: 338-342 (1959)). Another study found that, in the presence or absence of dietary calcium and magnesium, stearic acid was well absorbed by rats when esterified on the 2-position of triglycerides having oleic acid at the 1- and 3-positions, but absorption decreased when a second stearic was added to the 1-position (Mattson et al., J. Nutr. 109: 1682-1687 (1979), Table 3, page 1685). Stearic acid in the 1-position was well absorbed from triglycerides having oleic in the 2- and 3-positions in the absence, but not in the presence, of dietary calcium and magnesium (ibid.). With stearic acid in both the 1- and 3-positions, absorption decreased with or without dietary calcium and magnesium; the effect was more pronounced when calcium and magnesium were sufficient (ibid.).
The digestibility of palmitic acid has also been studied. Palmitic acid reportedly was better absorbed by rats when situated at the 2-position of triglycerides than at the 1- or 3-positions in naturally occurring fats commonly fed to human infants, and total fat absorption was adversely influenced by increasing the palmitic acid and stearic acid content in the 1- and 3-positions (Tomerelli et al., J. Nutr. 95: 583-590 (1968)).
While triglycerides high in stearic acid are less well utilized, they also tend to be high melting. Tristearin is a solid at room temperature; the alpha form is a white powder that melts at 55° C., which, on solidification, reverts to the beta form that melts again at 72° C. The melting points of 1,3-distearin with short or medium chain fatty acids at the 2-position have been indicated to be high (Lovegren et al., J. Amer. Oil Chem. Soc. 55: 310-316 (1978)). Symmetrical di-saturated triglycerides of stearic acid and/or palmitic acid, often with oleic acid at the 2-position, melt fairly uniformly near body temperature, and this property is advantageous for cocoa butter and hard butter substitutes (see, for example, U.S. Pat. Nos. 4,364,868, 4,839,192, and 4,873,109), and for hardstocks for margarines and shortenings (see, for example, U.S. Pat. Nos. 4,390,561, 4,447,462, 4,486,457, 4,865,866, and 4,883,684). Because of their functionality, high melting, high stearic fats generally have limited applications in food compositions as compared to more plastic or liquid triglycerides.
Fats have been prepared by substituting acetic acid for a portion of the fatty acids occurring in ordinary fats or oils, thus producing triglycerides bearing short acetyl and longer substituents. For saturated fats high in stearic acid, the substitution of acetyl groups for a portion of the stearyl groups lowers the melting point. These aceto-glycerides were investigated during the 1950's and found to be digestible. Animal feeding studies indicated that the nutritive value of mono- and di-acetin fats were essentially the same as the corresponding conventional triglycerides (Mattson et al., J. Nutr. 59: 277-285 (1956)), although acetooleins were more digestible than acetostearins (Ambrose et al., J. Nutr. 58: 113-124 (1956)). Animals grew poorly when fed acetostearin as the sole dietary fat (Coleman et al., J. Amer. Oil Chem. Soc. 40: 737-742 (1963)).
While lower melting than tristearin, acetostearins still have high melting points, limiting applications in food products requiring plastic or liquid fats. In fact, though melting points of structurally related compounds generally decrease with decreasing molecular weight (and mono- and distearins having medium to long saturated substituents follow this rule), the melting points of triglycerides in the C18CnC18 and CnCnC18 series, where n is 2 to 6, anomalously show the high molecular weight C6 (caproic acid) mono- and distearin derivatives to have the lowest melting points and the lower molecular weight C2 (acetic acid) mono- and distearin derivatives to have the highest (Jackson et al., J. Amer. Chem. Soc. 73: 4280-4284 (1951) and Jackson et al., J. Amer. Chem. Soc. 74: 4827-4829 (1952)). Plastic fats containing acetostearins suggested for use as shortenings and the like were formulated to contain significant levels of unsaturated fats and typically employed significant levels of fatty acids which would yield high saponification numbers or were liquid at room temperature (U.S. Pat. No. 2,614,937 and Baur, J. Amer. Oil Chem. Soc. 31: 147-151 (1954)).
Acetostearins are waxy fats having sharp melting points. In contrast to fats bearing medium and/or long substituents, acetostearins also exhibit unusual polymorphism (Feuge, Food Technology 9: 314-318 (1955)). Because of their melting and crystal properties, these fats have been suggested as useful for coating food products such as meat, fish, cheese, and candy (U.S. Pat. Nos. 2,615,159 and 2,615,160). Such compositions are often referred to as “hot melts” and may contain antibiotics (U.S. Pat. No. 3,192,057) or polymeric materials (U.S. Pat. No. 3,388,085) to prolong the life of the coating.
The short chain fatty acids (e.g., acetic, propionic, and butyric acid) or so-called volatile fatty acids, occur in the large intestine of mammalian species (Cummings, Gut 22: 763-779 (1981)). Except for a small percentage of butyric acid in milk fat (i.e., about 3.5 to 4 percent), volatile fatty acids rarely occur in nature esterified to glycerol in fats, but are generally by-products of fermentation in the gut. Physically, short chain fatty acids have been characterized as “not at all ‘fatlike’ in character; in fact they are hydrophilic substances with complete miscibility with water” (Bailey's Industrial Oil and Fat Products, 4th. Ed., J. Wiley, New York, 1979, volume 1, pages 16 to 17).
Early studies investigating the metabolism of short acids and triglycerides bearing short chain residues did not show a regular relationship between nutritional value and the number of carbon atoms in the fat (Ozaki, Biochem. Z. 177: 156-167 (1926) at 163). For example, when fed to rats at levels of 5 percent and 10 percent of the diet, triacetin and tributyrin were nutritious, yielding weight gains in the top 20 to 25 percent of the fats tested, whereas tripropionin and triisovalerin were toxic. In 1929, Eckstein reported that rats fed triolein and sodium butyrate grew at the same rate (J. Biol. Chem. 81: 163-628 (1929) at 622).
In 1935, Holt et al. observed that infants fed milk enriched with tributyrin retained more fat per day (90.1 to 90.2 percent) than those in a butterfat control group (88.9 percent); the study concluded that absorption was favored by fatty acids with relatively short chains (J. Ped. 6: 427-480 (1935), Table VIII, page 445, and Conclusions, number 4, page 477). Similar results were obtained with triacetin, with absorption of tributyrin and triacetin reportedly superior to that of corn oil, although corn oil yielded higher calorie level (Snyderman et al., Arch. Dis. Childhood 30: 83-84 (1955)). Substitution of triacetin, tripropionin, or tributyrin for half the glucose and starch in a rat diet did not significantly affect the digestible, metabolizable, or net energy measurements, but lower body weight gains were observed in animals fed tributyrin in two experiments and triacetin in one experiment (McAtee et al., Life Sci. 7: 769-775 (1968)).
Based on in vitro digestibility studies, tributyrin is readily cleaved by pancreatic lipase. Data measuring lipolysis as a function of chain length show tributyrin much more rapidly hydrolyzed than other substrates (see Sobotka et al., J. Biol. Chem. 105: 199-219 (1934), comparing triglycerides bearing three identical C4 to C18 acyl groups, and Desnuelle et al., J. Lipid Res. 4: 369-384 (1963), comparing triglycerides bearing three identical C2 to C18 acyl groups); some reports, however, rank tripropionin slightly better (Weinstein et al., J. Biol. Chem. 112: 641-649 (1936)), comparing triglycerides bearing three identical C2 to C6 acyl groups, and Wills, in The Enzymes of Lipid Metabolism, (Desnuelle, P., Ed.), Pergamon Press, N.Y., 1961, pages 13 to 19, comparing triglycerides bearing three identical C2 to C18 acyl groups). In fact, because tributyrin is such a good substrate and because the triglyceride is sufficiently water-soluble to allow enzymatic measurements in a homogeneous solution, it is often selected as a lipase substrate standard (Ravin et al., Arch. Biochem. Biophys. 42: 337-354 (1953) at 353).
Other lipase preparations readily cleave short chain triglycerides. Tributyrin was found to be hydrolyzed with the greatest initial velocity by human milk lipase, while pig liver lipase hydrolyzed tripropionin and tributyrin with an initial velocity much greater than any other in a study comparing C2 to C18 triglycerides (Schonheyder et al., Enzymologia 11: 178-185 (1943)). Tributyrin was hydrolyzed more readily than C6 to C18 triglycerides by human milk bile salt-activated lipase (Wang et al., J. Biol. Chem. 258: 9197-9202 (1983)). A liver lipase hydrolyzed trivalerin the fastest, with tributyrin the second fastest (Sobotka et al., J. Biol. Chem. 105:199-219 (1934)).
In contrast to triglycerides bearing long chain (about C16 to C24) fatty acids and those bearing short chain fatty acids, medium chain triglycerides, generally obtained from kernel oils or lauric fats and encompassing those substituted with C6 to C12, predominantly C8 to C10, fatty acids, have been of particular interest because they are more rapidly absorbed and metabolized, via a different catabolic route than those bearing long chain fatty acids (see Babayan, in Dietary Fat Requirements in Health and Development, (Beare-Rogers, J., ed.), A.O.C.S. 1988, Chapter 5, pages 73 to 86). Hence, medium chain triglycerides have been employed in premature infant formulas and in the treatment of several malabsorption syndromes. Feeding studies by Kaunitz et al., demonstrated the usefulness of medium chain triglycerides in weight maintenance and obesity control in rats (J. Amer. Oil Chem. Soc. 35: 10-13 (1957)).
Several research groups have exploited the physical and nutritional properties of medium chain fatty acids by suggesting that triglycerides having stearic and/or behenic acid in combination with medium chain substituents could be used as low calorie fats. See, e.g., European Patent Application Publication No. 0 322 027 B1 (May 19, 1993), (medium chain substituents defined as C6 to C10 residues), and Japanese Patent Publication No. 2-158, 695 (Jun. 19, 1990), (medium chain substituents defined as C4 to C12 residues). The latter publication, however, exemplified only trace amounts of C4 fatty acids, and suggested incorporating 0 to 1 long chain, unsaturated residues as well. Low calorie triglyceride mixtures having stearic acid at the 1-position and medium and unsaturated residues in the other positions have also been suggested (U.S. Pat. No. 4,832,975).
The polymorphism of triglycerides bearing medium and long moieties generally resemble fats bearing long moieties in that they tend to have a stable beta crystal structure. This contributes to graininess of fat mixtures containing them, and to the appearance of bloom in chocolate compositions. The preparation of smooth blends require careful substituent selection and/or tempering. It would be desirable to have low calorie fat mixtures free of this disadvantage. It would also be desirable to have a fat which was a true triglyceride but which delivered a minimum of calories and exhibited functionalities which permitted use in a wide variety of products.
U.S. Pat. No. 6,369,252 describes structured lipids and mixtures thereof and enzymatic methods with lipase for forming them. According to this patent, the rate of autoxidation and melting properties of triacylglycerols can be affected by the position of unsaturated fatty acids in the triacylglycerol molecule. Thus, triacyl-glycerols having unsaturated fatty acids at the 2-position are more stable toward oxidation than those linked at the 1- and 3-positions. U.S. Pat. No. 6,277,432 describes plastic fat compositions based on mixtures of triacylglycerides having various combinations of short, saturated long chain, and unsaturated long chain fatty acid residues in which 40-95 percent of the mixture comprises di-short chain species and 3-40 percent of the fatty acid moieties, including ones at the 2-position, are unsaturated long chain.
U.S. Pat. No. 5,380,544 describes novel triacylglycerol compositions containing at least 24 percent of novel structures with a short (S), medium (M), and long (L) chain saturated fatty acid randomly distributed among the three available triacylglycerol positions. The compositions belong to a class of reduced calorie fats useful for the formulation of table spreads and other food products. The random arrangement of fatty acids with respect to glycerol backbone of the triacylglycerols has been confirmed. (See, for example, Klemann et al., J. Agric. Food Chem., “Random Nature of Triacylglycerols Produced by the Catalyzed Interesterification of Short- and Long-Chain Fatty Acid Triglycerides,” 1994, 42, 442-446.)
The literature also identifies high melting mono-glyceride and diglyceride by-products of digestion as a source of decreased stearic acid bioavailability. Dreher et al., “Salatrim: A Triglyceride-Based Fat Replacer,” Nutrition Today, 1998, 33, 164-170. The literature additionally describes research to develop an empirical relationship between caloric availability and triacylglycerol compositions. Klemann et al., “Estimation of the Absorption Coefficient of Stearic Acid in Salatrim Fats,” J. Agric. Food Chem., 1994, 42, 484-488. An enhanced rate of cleavage of short chain acids relative to long chain fatty acids in structured triacylglycerols containing both short and long chain acids has been reported (Hayes et al., “In Vivo Metabolism of Salatrim Fats in the Rat,” J. Agric. Food Chem., 1994, 42, 500-514). The digestion and absorption of structured lipids containing medium- and long-chain fatty acids has been described in the literature. Fat Digestion and Absorption, Chapter 11, “Digestion and Absorption of Structured Lipids,” pp. 235-243, AOC Press 2000. The use of lipid structure components to reduce available caloric content of a food ingredient has been suggested. Structured and Modified Lipids, Chapter 18, Auerbach, et al., “Reduced-Energy Lipids”, Marcel Dekker, pp. 485-510. The literature has also suggested that certain commercial cooking oil products containing 80 percent diacylglycerol structure from soy and rapeseed are less likely than other oils to deposit as excess adipose tissue in the body. See, e.g., Jago, “Health Begets Wealth,” Prepared Foods, 21-22, April 2001. A method to determine the bioavailable energy from fat or oil also has been described. Finley et al., “Growth Method for Estimating the Caloric Availability of Fats and Oils,” J. Agric. Food Chem., 1994, 42, 489-494. Some commercially available synthetic fats and oils have been touted as delivering calorie reduction, or weight management, or health benefits, as mutually exclusive properties.